1. Field of the Invention
The present invention relates to a muffler for electrical equipment enclosures, and in particular, to a muffler for enhanced arc protection configured to exhaust arc gas and plasma away from electrical equipment.
2. Description of Related Art
Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit breakers and other switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, and receive electrical power from a power source and distribute the electrical power through one or more feeder circuits to one or more loads. Switchgear enclosures typically include circuit protection devices for interrupting electric power in a particular feeder circuit in response to hazardous current overloads in the circuit. A circuit protection device in electrical equipment can be a circuit breaker, fuse and switch combination, contactor and fuse combination or any other device intended to break or protect the load or secondary side of a circuit.
Switchgear or switchgear cabinet is a general term for an electrical enclosure encompassing switching and interrupting devices and their combination with associated control, instruments, metering, protective and regulating devices, and assemblies of these devices with associated interconnections, accessories, and supporting structures used primarily in connection with the generation, transmission, distribution, and conversion of electric power. Switchgear characteristics are described in ANSI/IEEE (American National Standards Institute/Institute of Electrical and Electronics Engineers) Standard No. C37.20.1, C37.20.2, C37.20.3-1999. However, embodiments of the present invention can be used in many other types of electrical equipment where arc resistance is required.
The specified temperature limits applicable to switchgear assemblies are given in the above referenced standards. The rated continuous current of metal-enclosed (ME) switchgear is the maximum current that can be carried continuously by the primary circuit components, including buses and connections, without producing a temperature in excess of specified limits for any primary or secondary circuit component, any insulating medium, or any structural or enclosing member. The continuous current ratings of the main bus in ME switchgear are also defined by the above referenced standards. The short-time current ratings of the individual circuit-breaker compartments of ME switchgear are equal to the short-time ratings of the switching and protective devices used, or the short-time rating of the current transformers (see ANSI/IEEE C57.13-1993).
In addition to current overloads, switchgear enclosures may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current “arcs,” flowing through ionized gas between conductors, such as between two ends of broken or damaged conductors, or between a conductor and ground in a switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, etc. During an internal arc event in an electrical enclosure, conductors and surrounding material are vaporized and converted to plasma and arc gas. The massive rate of expansion of the plasma and arc gas generates high pressure inside the equipment. As a result of the high pressures generated, the door/cover, hinges and latching means are placed under significant stress. If the door/cover, hinges and latching means should yield and deform, personnel outside the equipment would be exposed to dangerous levels of energy. In other words, the ionized gases associated with arcing faults may be released at pressures and temperatures sufficient to damage the switchgear equipment and cause deadly harm to anyone in close proximity.
Presently, the most commonly employed method for enhancing the durability of switchgear enclosures in the event of arcing faults is to provide arc-resistant switchgear that meets switchgear standards, with a means for venting the gases from the compartment in which an arcing fault occurs. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to the switchgear. This control of the explosion exhaust is what provides the increased safety to personnel working around the equipment. In some instances, electrical equipment contains components that require external ventilation openings in the equipment enclosure. During an arc event in such an enclosure, the ventilation openings provide a means for the dangerous energy to propagate outside the equipment, and could provide exposure to personnel standing in front of the equipment.